1. Field of the Invention
This invention relates to temperature sensing systems, and more particularly to printed circuit boards for use with thermocouples and similar devices.
2. Description of the Related Art
Thermocouples are often employed as temperature sensors to acquire temperature measurements in a number of test and measurement environments. For example, a thermocouple may be placed inside of a heating or cooling chamber to assess and determine a temperature inside the chamber. Thermocouples measure the temperature difference between two points, not absolute temperature. To measure a single temperature one of two of the junctions of the thermocouple (e.g., the cold junction—typically located near a device reading the thermocouple) is maintained at a known reference temperature, and the other of the two junctions (e.g., the hot junction) is provided at the location for sensing temperature. By determining the relative temperature difference between the hot and cold junctions and using a known or estimated reference temperature at the cold junction, a temperature at the hot junction can be determined. That is, the unknown temperature at the hot junction is determined based on a known/estimated cold junction temperature and a relative difference in temperature between the hot and cold junctions.
Determining the actual temperature at the cold junction is not always possible or practical. For example, it may be difficult or impractical to assess a cold junction temperature at each of a plurality of input terminals of a terminal block each coupled to a thermocouple. Often, applications provide a best estimate of the cold junction temperature by using a thermally sensitive device, such as a thermistor, to measure the temperature of the input connections at or near one or more terminals coupled to thermocouples. For example, where leads of a thermocouple are connected to a terminal block on a printed circuit board (PCB), a thermistor may be placed on the PCB at or near the terminal block to estimate the cold junction temperature at the terminal block. In some instances, a single thermally sensitive device may be used to estimate a single cold junction temperature for multiple terminals located near the thermally sensitive device. For example, a single thermistor may be placed adjacent a row of terminal connectors, and measurements from the single thermistor may be used to assess and estimate a cold junction temperature at all of the terminal blocks near the thermistor. Based on the temperature estimates at the thermistor, the cold junction temperature and associated voltage at the cold junction can be estimated, and the appropriate correction applied to determine a corresponding sensed temperature at the hot junction. This is known as cold junction compensation.
Unfortunately, the cold junction temperature determined using thermally sensitive devices as described above may be inaccurate by several degrees or more. For example, a temperature gradient may exist across several terminals such that the cold junction temperature at one terminal is not the same as the cold junction temp at another terminal and/or the nearby thermally sensitive device (e.g., thermistor). As a result, using the cold junction temperature acquired via the thermally sensitive device may be inaccurate for some or all of the terminals. Often, the cold junction temperature is increasingly inaccurate for terminal bocks that are located farther away from the thermally sensitive device due to temperature gradients between the thermally sensitive device and the terminal blocks. It will be appreciated that using such an inaccurate cold junction temperature may result in inaccurate cold junction compensation and, thus, in inaccurate assessments of the temperature at the hot junction.
Accordingly, it is desirable to provide a technique for acquiring accurate cold junction temperature information for thermocouples.